Plant cells and tissues can respond to mechanical, chemical or pathogen induced injury by producing various phenolic compounds including mono- or dimethoxylated lignin precursors derived from cinnamic acid via a complex series of biochemical reactions. These lignin precursors are eventually used by the plant to produce the lignin polymer which helps in wound repair by adding hydrophobicity, a physical barrier against pathogen infection and mechanical strength to the injured tissue (Vance, C. P., et al., 1980, Annu Rev Phytopathol 18:259–288). Biosynthesis of the mono- or dimethoxylated lignin precursors occures, in part, by the action of two enzymes, caffeic acid 3-O-methyltransferase (COMT), also known as caffeic acid/5-hydroxyferulic acid O-methyltransferase and caffeoyl CoA 3-O-methyltransferase (CCOMT). Both enzymes have been isolated and purified from a wide variety of plant species.
Studies have shown that the activities of COMT and CCOMT increase prior to lignin deposition (Inoue, K., et al., 1998, Plant Physiol 117(3):761–770). Synthesis of lignin precursors involves the methylation of caffeic acid to yeild ferulic acid followed by 5-hydroxylation of ferulate then a second methyltion to yield sinapate. COMT has been implicated in the methylation of both caffeic acid and 5-hydroxyferulic acid ((Inoue, K., et al., 1998, Plant Physiol 117(3):761–770). Research indicates that COMT transcripts are present at high levels in organs containing vascular tisssue and one study suggests that antisense inhibition of COMT can lead to modified lignin content and composition in the xylem and phloem of transgenic plant tissue (Dwivedi, U., et al., 1994, Plant Mol. Biol. 26:61–71).
Because of lignins importance in cell wall architecture and wound repair mechanisms there is considerable interest. in the prospects for altering lignin quantity or quality by genetic engineering. For example, chemical treatments needed to remove lignin during the paper-pulping process are expensive and environmentally unfriendly. Plants with altered lignin quantity or quality could benefit this industry (Boudet, A., et al., 1996, Mol Breeding 2:25–39; Campbell, M., et al., 1996, Plant Physiol 110:3–13). Thus, there is a great deal of interest in identifying the genes that encode proteins involved in the production of lignin in plants. These genes may be used in plant cells to control lignin production. Accordingly, the availability of nucleic acid sequences encoding all or a portion of an enzyme involved in the production of lignin would facilitate studies to better understand lignin production in plant cells and provide genetic tools to enhance or otherwise alter lignin biosynthesis which in turn could provide mechanisms to control cell wall architecture and host defence and injury repair mechanisms in plant cells.